There is a major unresolved challenge in recycling mixed plastics and mixed plastics with non-plastic-contaminates. The difficulty with this type of recycling process is that the different plastics used in the recycling process are not compatible for use or combination with each other. There is an inherent inability of two or more dissimilar plastics to undergo mixing or blending, which means that some plastics cannot be mixed together during the recycling process. Also, different types of plastics are immiscible at the molecular level, and there are significant differences in processing requirements at the macroscopic level with respect to the different plastic inputs.
Even a small amount of the wrong type of plastic can make it impractical to recycle an entire container or bale of reclaimed material. Co-mingled reclaimed material is worth significantly less than sorted material. Mixed or co-mingled plastics are frequently contaminated with such items as metals, paper, pigments, inks, adhesives, carbon fiber, flame retardants, fiber reinforced plastics, glass filled plastics, cured silicon and rubber.
A primary reason for the incompatibility of mixed reclaimed materials is the different melting points associated with different plastic resins and the inability of certain plastics to undergo a “re-melting” process. Plastics involved in recycling activities can be considered in two broad categories: thermoplastics and thermoset. Comparing these types, in the present art, thermoplastics are much easier to adapt to recycling.
Thermoplastic polymers can be heated and formed, then heated and formed again and again. The shape of the polymer molecules are generally linear or slightly branched. This means that the molecules can flow under pressure when heated above their melting point. Thermoset polymer plastics, on the other hand, undergo a chemical change when they are heated, creating a three-dimensional network. Thermosets cannot be re-melted or remolded and therefore have been traditionally difficult to recycle. Typical types of thermosetting plastics are Polyurethane (PU), epoxies, polyesters, silicones and phenolics, with vulcanized rubber being an excellent example of a thermosetting and also polyoxybenzylmethylenglycolanhydride (bakelite).
Materials made from two polymers mixed together are called blends. In general, polymers cannot be homogeneously mixed with one another, and even attempting to mix most polymers will result in phase-separated mixtures. An example of phase-separated mixtures or immiscible blends is polystyrene and polybutadiene, which are immiscible polymers. When you mix polystyrene with a small amount of polybutadiene, the two polymers will not blend together. Polystyrene is generally a stiff, brittle, material that will break or shatter if bent. Polybutadiene separates from the polystyrene into usually small, isolated, sphere-shaped items, and the polybutadiene spheres in the blend are elastic in nature and absorb energy under stress.
The polystyrene and polybutadiene immiscible blend bends and does not break like polystyrene by itself. The immiscible blends of polystyrene and polybutadiene are known as high-impact polystyrene, or HIPS. Another example of an immiscible blend is one made from Polyethylene Terephthalate (PET) and poly(vinyl alcohol) (PVA). The blend results in PET and PVA separating into individual sheetlike layers. This blend is particularly useful in the making of plastic bottles for carbonated liquids.
“Recycling and Recovery of Plastics,” by Joop Lemmens recognizes a recent trend in the increased use of polymer mixtures, blended polymers and novel plastic combinations. An example of ‘novel’ plastic is cross-linked polyethylene (PEX). Crosslinking polyethylene changes the polymer from a thermoplastic to a thermoelastic polymer. Once it is fully crosslinked, polyethylene tends not to melt but merely to become more flexible at higher temperatures.
Examples of problems in recycling plastics include cases where a quantity of recycled PET is contaminated with a small amount of PVC. The PVC will release hydrochloric-acid gas before the process temperature to melt the PET is reached, and the released gas will degrade the PET. In the reverse, where a small amount of PET contaminates recycled PVC, the PET will remain in solid form after the PVC reaches its melting point, which results in crystalline PET inhabiting the post-melt-cooled PVC structure.
A major problem in the recycling of PVC is its high chlorine content of raw PVC, and the hazardous additives added to the polymer to achieve the desired material quality. PVC requires separation from other plastics and sorting before mechanical recycling. PVC recycling is difficult because of high separation and collection costs, loss of material quality after recycling, and the low market price of recycled PVC compared to virgin PVC.
There are thousands of different varieties of plastic resins or mixtures of resins, and most plastics have a code number or classification. Plastics not identified by code numbers are difficult to recycle. These items, such as computer keyboards, do not fit into the numbering system that identifies plastics used in consumer containers.
Bazant & Cedolin address concerns about the stability of structural composites, such as the composites made from recycled materials. Namely, Bazant & Cedolin states three-dimensional instabilities are important for solids with a high degree of incremental anisotropy, which can be either natural, as is the case for many fiber composites and laminates, or stress-induced, as is the case for highly damaged states of materials and the typical three-dimensional instabilities are the surface buckling and internal buckling, as well as bulging and strata folding.
Bazant & Cedolin state that three dimensional buckling modes no doubt play some role in the final phase of compression failures. For example, Bazant (1967) showed that a formula based on thick-wall buckling agrees with his measurements of the effects of the radius-to-wall thickness ratio on the compressive failure stress of fiber-glass laminate tubes. On the other hand, other physical mechanisms, particularly the propagation of fractures or damage bands, are no doubt more important for the theory of compression failure. The reason is two-fold: (1) the calculated critical states for the three-dimensional instabilities require some of the tangential moduli to be reduced to the same order of magnitude as some of the applied stress components, which can occur only in the final stage of the failure process; and (2) the body at this stage might no longer be adequately treated as a homogeneous continuum.
Bazant & Cedolin address orthotropic composites that have a very high stiffness in one direction and a small shear stiffness may suffer three-dimensional instabilities such as internal buckling or surface buckling. These instabilities, which involve buckling of stiff fibers (glass, carbon, metal) restrained by a relatively soft matrix (polymer), are analogous to the buckling of perfect columns. When the fibers are initially curved, one may expect behavior analogous to the buckling of imperfect columns. In particular, the initial curvature of fibers causes fiber buckling, which reduces the stiffness of the composite. It also gives rise to transverse tensions, which may promote delamination failure.
Urquhart & O'Rourke address three dimensional instabilities as whenever a material is subjected to compression in one direction, there will be an expansion in the direction perpendicular to the compression axis. When this expansion is resisted, lateral compressive stresses are developed, which tend to neutralize the effect of the longitudinal compressive stress, and thus increase resistance against failure. This is the principle involved in the use of spiral or hooped reinforcement.
Also, Urquhart & O'Rourke states that within the limit of elasticity the hooped reinforcement is much less effective than longitudinal reinforcement. Such reinforcement, however, raises the ultimate strength of the column, because the hooping delays ultimate failure, and the material continues to compress and to expand laterally, thus increasing the tension in the bands, while final failure occurs upon the excessive stretching or breaking of the hooping. As long as the bond between the fiber and the polymer is effective, the two materials will deform equally, and the intensities of the stresses will be proportional to their moduli of elasticity.
A structural system's failure mode can be defined as the characteristics bounded by that known as catastrophic or localized within the said system, wherein the term catastrophic indicates a system-wide structural failure involving progressive individual and sub-systemic structural element(s) failures and the term localized indicates a system or sub-system arrest of structural failure and/or redistribution of the force(s) which resulted in the initial failure-mode of the initial failed structural element.
One of the unexpected results of full-scale testing of the present invention's physical manifestations, is the damping effect of the present invention to structural shock, such as the characteristic of nailing or directly impacting physical samples of the present invention.
U.S. Pat. No. 6,497,956 (956) issued Dec. 24, 2002, to Phillips et al., teaches that high density polyethylene (HDPE) and plastic lumber made from HDPE, PVC, PP, or virgin resins has been characterized as having insufficient stiffness to allow its use in structural load-bearing applications. For example, it is noted that non-reinforced plastic lumber products typically have a flexural modulus of only one-tenth to one-fifth that of wood such as Douglas fir. This process uses a laminar flow of material that is extruded in a melt extrusion process. U.S. Pat. No. 5,212,223 discusses the inclusion of short glass fibers within reprocessed polyolefin and further teaches doing so to increase the stiffness of the non-reinforced plastic lumber by a factor of 3:4. However, none of the prior art known to applicant is capable of fabricating plastic lumber having the structural stiffness and strength of products made according to the present invention.
U.S. Pat. App. No. 20070045886, filed Mar. 1, 2007, by Johnson teaches that composite lumber is currently used for decking, railing systems and playground equipment. Sources indicate that there currently exists a $300 million per year market for composite lumber in the United States. It is estimated that 80% of the current market uses a form of wood plastic composite (WPC). It is estimated that the other 30% is solid plastic. A wood plastic composite (WPC) refers to any composite that contains wood particles mixed with a thermaloset or thermoplastic. The presence of wood fiber increases the internal strength and mechanical properties of the composite as compared to, e.g., wood flour. And, for example, the addition of wood fillers into plastic generally improves stiffness, reduces the coefficient of thermal expansion, reduces cost, helps to simulate the feel of real wood, produces a rough texture improving skid resistance, and allows WPC to be cut, shaped and fastened in a manner similar to wood.
Also, the addition of wood particles to plastic also results in some undesirable characteristics. For example, wood particles may rot and are susceptible to fungal attack, wood particles can absorb moisture, wood particles are on the surface of a WPC member can be destroyed by freeze and thaw cycling, wood particles are susceptible to absorbing environmental staining, e.g., from tree leaves, wood particles can create pockets if improperly distributed in a WPC material, which may result in a failure risk that cannot be detected by visual inspection, and wood particles create manufacturing difficulties in maintaining consistent colors because of the variety of wood species color absorption is not consistent. Plastics use UV stabilizers that fade over time. As a result, the wood particles on the surface tend to undergo environmental bleaching. Consequently, repairing a deck is difficult due to color variation after 6 months to a year of sun exposure.
In a typical extrusion composite design, increased load bearing capacity capability may be increased while minimizing weight by incorporating internal support structures with internal foam cores. Examples of such designs are taught in U.S. Pat. Nos. 4,795,666; 5,728,330; 5,972,475; 6,226,944; and 6,233,892.
Increased load bearing capacity, stability and strength of non-extruded composites has been accomplished by locating geometrically shaped core material in between structural layers. Examples of pre-formed geometrically shaped core materials include hexagon sheet material and lightweight woods and foam. Problems associated with typical preformed core materials include difficulties associated with incorporating the materials into the extrusion process due to the pre-formed shape of the materials.
Other efforts to increase strength with composite fiber design have focused on fiber orientation in the composite to obtain increased strength to flex ratios. In a typical extrusion composite process, the fiber/fillers are randomly placed throughout the resin/plastic. Therefore increasing strength by fiber orientation is not applicable to an extrusion process.
Foam core material has been used in composites for composite material stiffening (e.g., in the marine industry) since the late 1930's and 1940's and in the aerospace industry since the incorporation of fiber reinforced plastics. Recently, structural foam for core materials has greatly improved in strength and environmental stability. Structural core material strengths can be significantly improved by adding fibers. Polyurethane foams can be modified with chopped glass fibers to increase flexible yield strength from 8,900 psi-62,700 psi.
Prior art patents tend to describe foam core materials as rigid or having a high-density. However structural mechanical properties of the foam core tend not to be addressed. A common method to obtain a change in load capacity is to change the density of the material. For example, this can be done in a polyurethane in which water is being used as a blowing agent. The density of a polyurethane decreases with the increase in water concentration.
One problem that may occur when a core material and a structural material are not compatible both chemically and physically is delamination. Chemical and physical incompatibility can result in composite structures that suffer structural failures when the core material and the structural material separate from one another.
Coefficient of thermal expansion (CTE) is discussed in Johnson, as well as the conformable core material is injected into and around internal structural support members of an extruded member. Preferably, while the member is being extruded, the core material is injected to replace air voids within the member. The injection of conformable structural core material at the same time and same rate as the structural member is being extruded produces significant improvements by increasing load bearing capacity, stability and overall strength and by improving economic feasibility. For example, a rigid polyurethane foam is approximately 10 times less expensive per volume than PVC. Therefore, by replacing some interior volume of an extruded member with foam, the PVC volume is reduced while maintaining the same structural strength or greater. Therefore, the injection of a conformable foam results in a significant cost savings. In some applications, the injectable conformable structural core material may be applied to an extruded member that has been previously cured.
One benefit of an injectable conformable structural core material is that the core material is not limited by the structural design of the composite member because the core material conforms to the geometric shapes present in structure.
Although a core material and a structural material may be initially combined into a composite member without regard to the CTE's of each, this does not guarantee structural integrity over time. Therefore, the invention of the application involves tailoring of the conformable structural core material by the selection of optimal amounts of structural fillers to achieve a desired CTE of the materials. The step of tailoring the structural core material provides a solution for composite structural design regardless of the composition of the materials.
One aspect of the invention is directed towards the mechanical interaction and the relationship between a selected thermal plastic and a selected foam core material. Thermal plastics have mechanical properties that are influenced by environmental temperatures. For example, thermal plastics are stronger at colder temperatures but are more brittle. Thermal plastics are weaker in warmer weather, but are more flexible.
Foam for an internal core material inside a thermal plastic material may be tailored to overcome variations in structural strengths of thermal plastics. For example, an ideal core material is selected to possess thermal expansion properties that offset the thermal sag characteristics of thermal plastic structural material that the structural material experiences due to thermal heating in the environment. The thermal expansion of the core and mechanical stiffness of the composite may be tailored to achieve desired strength and internal pressure, resulting in mechanical stiffening of the composite.
The interaction of thermal sag of the thermal plastic material in relationship to the thermal expansion of the internal core material may be considered to select an ideal foam for use with a particular plastic. Ideally, the materials will function as a true composite. Because of the enormous uses of this invention associated with composite design and their applications with the overwhelming selection of materials and their combinations, the method described herein allows for optimal material pairings to be determined. As internal cross members of a structural member and the exterior structure undergo mechanical weakening as the temperature increases, a selected internal core material having an optimal thermal expansion with enhanced thermal mechanical properties will improve the rigidity and the mechanical strength of the combined composite in a manner similar to inflating an automobile tire to increase mechanical rigidity of the rubber.
A further advantage associated with the use of core materials such as foams are thermal insulation properties of the foam. A significant mechanical advantage is achieved by reducing the heat transfer rate from the surface of a structural member to an internal support structure of the composite, thereby thermally shielding the internal support structure from heat fluctuations and maintaining increased internal strengths of the cell structures in the composite during elevated temperatures.
CTE can be tailored in a composite matrix to improve surface functionality between the structural material and the core, thereby reducing the shear stresses that are created by thermal cycling at the contact interface of the two materials. Polyurethane foam densities are directly proportional to the blowing agent, typically water. The less water, the tighter the cell structure, which results in higher density foams.
In a closed cell structure, controlling internal forces caused by thermal cycling produced by the core material can be accomplished by tailoring the CTE. The CTE of a core material may be tailored by adjusting an amount of filler in the core material. For example, fillers such as chop fibers and micro spheres will have much lower CTE in the structural foam. The CTE of glass spheres is approximately 100 times smaller than most resin materials.
Glass spheres or ceramic spheres have enormous compression strength in comparison to the foam cells created by blowing agents. Therefore, the addition of micro spheres will not only provide the ability to tailor the CTE of the foam but it will replace low compression strength cell structures with higher strength cell structures.
The incorporation of chop fibers adds dramatic cross structural strength throughout the foam. Applicant's mechanical model analysis clearly illustrates an increased strength of materials resulting from the presence of core material regardless of the mechanical structure. The analysis was directed to extruded PVC. Some of the extruded PVC members were filled with chopped fibers and some were not. The chopped fibers increased strength of the structural member and decreased the CTE. The additives of selected fillers to the foam core materials illustrate similar characteristics. Selecting appropriate materials for a composite is complicated because composites are not homogeneous materials. However, composites are required to function as a homogeneous structure without structural deviation. The models clearly show how reinforcing fibers increases load bearing capabilities in the composite materials.
Manmade fibers and fillers can be used to improve mechanical properties as well as to lower CTE's of a core material. Ideally, filler materials should be environmentally stable and malleable into desired geometric configurations so that they may be incorporated into a structural design. Examples of fiber materials include fiberglass, carbon and nylon. These fibers can be cut to a specific length with a desired diameter that can be incorporated into an injection molding process either from the plastics manufacturer if the desired material is a foam plastic. If the resin is a reactive material such as polyurethane foam, the fillers and fibers can be combined either in the liquid stage prior to mixing the reactive components or in the foam mixing chamber prior to being extruded. The coefficient of thermal expansion is directly related to the volume fillers to plastics ratio.
Solid core materials can be made from high-density polyurethane, polyureas and epoxy materials etc., having high strength and fast cure times. These materials may be filled with fillers or micro spheres to produce high strength injectable core materials.